1. Field of the Invention
This invention relates to a magnetic field adjustment device and a magnetic field adjustment method for a superconducting magnet as are chiefly employed for a magnetic resonance imaging equipment (hereinbelow, abbreviated to “MRI equipment”).
2. Description of the Related Art
In an MRI equipment, a static magnetic field which has a high magnetic field strength and which is of high spatial uniformity and high temporal stability is needed, and hence, a superconducting magnet is often employed.
It is required of the superconducting magnet for the MRI equipment to generate a magnetic field region which exhibits a very high uniformity of, for example, within 3 ppm, near a magnetic field center. Therefore, the superconducting magnet is designed at a high precision. In actuality, however, a magnetic field uniformity in an actual diagnostic space worsens due to a manufacturing dimensional error in a process for producing the magnet, the influence of a magnetic material existing around a place where the magnet is installed, and so forth.
Therefore, the superconducting magnet includes means for making the fine adjustment (hereinbelow, termed “shimming”) of a magnetic field. There is a passive shimming method in which magnetic material shims made of a magnetic material of high permeability are used as one means for performing the shimming. This method is such that the magnetic material shims in an appropriate quantity are arranged at an appropriate position, within the generated magnetic field of the superconducting magnet, and that the disorder of the highly uniform magnetic field of the superconducting magnet is corrected by utilizing a magnetic field generated by the magnetization of the magnetic material shims. (Refer to Patent Document 1 being JP-A-1-245154, and Patent Document 2 being JP-A-63-122441.)
The shimming based on a prior-art passive shimming method will be described with reference to FIGS. 13 and 14. FIG. 13 is a cut-away perspective view of a superconducting magnet for an MRI equipment as includes a prior-art magnetic field adjustment device. Referring to FIG. 13, numeral 21 designates the superconducting magnet which has a substantially cylindrical shape. Inside the superconducting magnet 21, a plurality of superconducting coils 22 are disposed substantially concentrically with the cylinder of the superconducting magnet 21, and they generate a static magnetic field in a uniform magnetic field space 23 to which a highly uniform magnetic field is to be outputted. The superconducting coils 22 are coils each of which is fabricated by winding a superconducting wire, and they are accommodated in a low-temperature container 24, together with liquid helium (not shown) which is a coolant required for holding these superconducting coils 22 in a superconducting state. The low-temperature container 24 is configured of a helium tank 25 in which the liquid helium and the superconducting coils are accommodated, a thermal shield 26 which serves to intercept thermal invasion from outside, and a vacuum tank 27 which holds the interior of this low-temperature container 24 in a vacuum state. Usually, a refrigerator (not shown) is connected to the low-temperature container 24 in order to suppress the consumption of the liquid helium.
Numeral 98 designates a magnetic material shim mechanism, which is fixed to the inside cylinder of the superconducting magnet 21. The magnetic material shim mechanism 98 is a mechanism for accommodating magnetic material shims therein so as to arrange them at the periphery of the uniform magnetic field space 23.
FIG. 14 shows the magnetic material shim mechanism 98. The magnetic material shim mechanism 98 is configured of the plurality of magnetic material shims 101, a shim tray 102 which accommodates the magnetic material shims 101 therein and which is fixed to the inside cylinder of the superconducting magnet 21, and shim spacers 103 and lids 104 which fix the magnetic material shims 101 within shim pockets. The shim tray 102 is formed of a nonmagnetic material, and it has a shim pocket structure for arranging the magnetic material shims 101 therein.
The magnetic material shims 101 are thin plates made of a magnetic material, for example, iron, and they have predetermined vertical and lateral dimensions so as to be accommodated in shim pockets. As the magnetic material shims 101, a plurality of sorts having different thicknesses are prepared. The quantity of the magnetic material which is arranged in each shim pocket is determined by the thicknesses and number of the magnetic material shims 101.
The shim tray 102 is formed with slits so that the lids 104 of snug fit type can be fixed. After the accommodation of the magnetic material shim 101 in the shim pocket, the shim spacers 103 made of a nonmagnetic material are packed into the shim pocket, and the lid 104 made of a nonmagnetic material is put on the shim pocket, whereby the magnetic material shims 101 are fixed within the shim pocket. The shim tray 102 has a mechanism for being fixed to the inside cylinder of the superconducting magnet 21, and it is fixed to the inside cylinder of the superconducting magnet 21 after the accommodation of the magnetic material shims 101. In this way, the magnetic material shims 101 are arranged on the magnet inside cylinder so as to correct the disorder of the magnetic field uniformity. The magnetic material shim mechanisms sometimes form part of the structure of inclined magnetic field coils in the MRI equipment.
The shimming which uses the magnetic material shim mechanisms is carried out by the following steps:
Initially, the superconducting magnet 21 is excited in a state where the magnetic material shims 101 are not mounted, and the magnetic fields of the uniform magnetic field space 23 are measured at a large number of points, so as to evaluate the uniformity of the magnetic field which the superconducting magnet 21 generates. In general, magnetic field strengths in a uniform magnetic field region are expressed as Formula (1) by employing a Legendre function expansion, and they are nominated by components based on (m, n) values. A (0, 0) component is a necessary uniform magnetic field component, and all the others are error magnetic field components which are nonuniform within the imaging region.
Subsequently, the arrangement of the magnetic material is designed in order to correct the nonuniformity of the magnetic field. The design is performed by a computation which optimizes the quantities of the magnetic material as is to be arranged in the respective shim pockets, so as to lessen the error magnetic field components to the utmost, on the basis of the measured magnetic fields at the large number of points. The result of the computation is outputted as a table in which the numbers of the magnetic material shims 101 to be attached into the respective shim pockets are listed. An operator attaches the magnetic material shims 101 on the basis of the table. When the attachment of the magnetic material shims 101 has been completed, the superconducting magnet 21 is excited and the magnetic fields of the uniform magnetic field space 21 are measured again, so as to reevaluate the uniformity of the generated magnetic field after the magnetic material shim correction.
Usually, it is difficult to bring the magnetic field uniformity into a target value by one time of execution. Therefore, the operations as stated above are repeated several times, whereby the magnetic field uniformity is gradually enhanced.
                                          B            z                    ⁡                      (                          r              ,              θ              ,              ϕ                        )                          =                              ∑                          m              =              0                        ∞                    ⁢                                          ⁢                                    ∑                              m                =                0                            n                        ⁢                                                  ⁢                                          r                n                            ⁢                                                P                  n                  m                                ⁡                                  (                                      cos                    ⁢                                                                                  ⁢                    θ                                    )                                            ⁢                              {                                                                            A                      n                      m                                        ⁢                                          cos                      ⁡                                              (                                                  m                          ⁢                                                                                                          ⁢                          ϕ                                                )                                                                              +                                                            B                      n                      m                                        ⁢                                          sin                      ⁡                                              (                                                  m                          ⁢                                                                                                          ⁢                          ϕ                                                )                                                                                            }                                                                        (        1        )            where:
Bz denotes a magnetic field at coordinates (r, θ, φ),
the coordinates (r, θ, φ) are indicated in FIG. 15, and
Anm and Pnm denote those coefficients of respective components which are determined depending upon a magnetic field shape.
With the prior-art magnetic material shim mechanism as stated above, the shim tray is a unitary molded article. Accordingly, there has been the problem that the positions of the magnetic material shims cannot be finely adjusted, and that the versatility of the magnetic material shim arrangements is low. Therefore, in a case where the uniformity of the magnetic field (hereinbelow, termed “rough magnetic field”) generated by the superconducting magnet itself is inferior, it has been sometimes impossible to make the uniformity correction by the prior-art magnetic material shim mechanisms. Besides, the shim tray designed optimally for the rough magnetic field characteristic of a specified superconducting magnet type cannot be diverted to the uniformity adjustment of a superconducting magnet type having another rough magnetic field characteristic. It has accordingly been necessary to design the optimal shim tray every superconducting magnet type.